Treating chronic pain is an important task of modern medicine. Chronic pain is a major public health problem in this country and is the cause of much physical and emotional disability. Pain may be roughly classified as being either acute or chronic. Acute pain, such as occurs after surgery or trauma, comes on suddenly and lasts for a limited time. Acute pain is a direct response to disease or injury to tissue, and typically subsides when the disease or injury is adequately treated. Chronic pain, on the other hand, is pain that persists for a long period of time, sometimes even after a known precipitating cause no longer exists. Common types of chronic pain include back pain, headaches, arthritis, cancer pain, and neuropathic pain resulting from injury to nerves.
Treating chronic pain may involve a number of different therapies such as physical rehabilitation, psychological counseling, and medications. Electrical stimulation of nervous tissue has also been found to be effective in treating certain kinds of chronic pain. Prior methods of delivering such neural electrical stimulation to relieve pain have utilized electrodes placed on the skin surface to stimulate underlying nervous tissue. Delivering neural electrical stimulation transcutaneously in this fashion, however, can only stimulate nervous tissue located at a relatively shallow depth beneath the skin surface.